Above - all good comments.
Here are my set of "Rules". I'm sure others have their own....
Up wind, above 18-20kts, leeway, rig loading, sheet/winch load, ride, heal and comfort are all compromised without taking a reef. In doing so, you will suffer no significant change in boat speed with improved VMG. Rule
1: Reef upwind by 20 kts., be preparing to reef at 18 kts. (When upwind use apparent wind speed)
On a reach: Loads will be less but apparent wind will be higher. The boat may well approach hull speed
so the likelihood of any significant gain from having more canvas
becomes moot. Generally, I'll again be looking to reef at 20 kts. perhaps with gusts to 25-26, always being mindful of not being caught out by abrupt increases in wind speed. You can always bear off and drop apparent wind, but sooner or later you have to come up to put in the reef. Rule 2. On a reach, reef near 20 kts. apparent, perhaps a few knots higher is the sea state is kind.
Down wind: This is where I've been caught out in the past. You must watch true wind speed as your apparent will be deceptively low. Certainly if you start to surf repeatedly, reef. I would generally be well reefed with a true wind of 20 kts. unless well crewed with a kind sea state where speed was key. Rule 3: Down wind reef at 20 kts. of true wind.
The reality is the hulls of boats like my Lagoon
are very buoyant in the bows, and the rig is very strong, so being caught out with say 30 kts. and a full main has never been a real problem, but one tries not to rely on safety
margins. Generally reefing will make the ride more comfortable, bring a genuine sense of confidence and reassurance and in a non surfing hull, your boat speed and VMG will not suffer. Rule 4: Reef early, not late.